1. Field of the Invention
This invention relates, in general, to the catalytic reforming of a gasoline boiling range petroleum feedstock to improve the octane rating thereof, and more, particularly, to an activation treatment of the reforming catalyst.
2. The Prior Art
Catalytic reforming is a process well known in the petroleum refining industry for improving the octane quality of naphthas and straight run gasolines. In a typical process, a series of reactors are provided with fixed beds of catalyst which receive upflow or downflow feed of naphtha and hydrogen concurrently passed through a reheat reactor, and then, in sequence, through subsequent heaters and reactors of the series. The vapor effluent from the last reactor of the series, a gas rich in hydrogen, is separated from the C.sub.5.sup.+ liquid product and small amounts of gaseous hydrocarbons and recycled to the process to minimize coke formation; coke invariably forming and depositing on the catalyst during the reaction.
Because of the demand for high octane for use as motor fuel, extensive research is being devoted to the development of improved reforming catalysts. Catalysts for successful reforming processes must possess good selectivity, i.e., be able to produce high yields of high octane number C.sub.5.sup.+ liquid products and, accordingly, low yields of light gaseous hydrocarbons and carbonaceous by-products. A small increase, e.g. 1-2% increase, in the C.sub.5.sup.+ liquid product yield can represent a large increase in the productivity of the catalytic process. For example, a 1.25% volume increase in C.sub.5.sup.+ liquid product yield is equivalent to an additional $600,000 earning gain per year for a 30,000 barrels per stream day (BSAD) catalytic reformer unit. To be commercially acceptable, the reforming catalysts must also possess good activity in order that the temperature required to produce a high C.sub.5.sup.+ liquid yield not be too high. Apart from good selectivity and activity, it is also necessary that catalysts possess good stability in order that the activity and selectivity characteristics be retained for prolonged use, e.g. 365 days.
Reforming catalysts are recognized as being dual functional. The catalyst is generally a composite including a metal, or metals, providing a hydrogenation- dehydrogenation (hydrogen transfer) function and an acidic component provides an isomerization function. The platinum group metals (ruthenium, osmium, rhodium, iridium, palladium and platinum), particularly platinum, have been widely used in commercial reforming operations, these metals being composited with an inorganic oxide base, particularly alumina. In recent years, promoters such as iridium, rhenium, germanium and tin, have been added, particularly to platinum, to enhance one or more of certain of the characteristics which a good reforming catalyst must possess, namely, activity, selectivity and yield stability. Rhenium has been found particularly useful in providing excellent C.sub.5.sup.+ liquid product yields and stability. Halogen, e.g. chlorine, is generally added to provide the required acid function.
It is believed that the chief benefit imparted by rhenium to the platinum catalyst is that of greater coke tolerance. The mechanism which makes the catalyst more tolerant of the deactivating coke is not known with any degree of certainty, but it is believed that the formation of intermetallic catalytic alloys, or bimetallic clusters, is the principal factor and that the formation of these species is dependent on the existence thereof of rhenium and platinum in the zero valent state. Note in this regard, a series of articles and Letters to the Editor in the Journal of Catalysis: 35, 434-440 (1974); 39, 485-486 (1975); 43, 18-33 (1976); 46, 438-440 (1977); 52, 444-456 (1978); 56, 468-471 (1979) and 59, 434-445 (1979). In some of these articles, the difficulties associated with reducing rhenium to the zero valent state are discussed and many studies have been conducted with the objective of achieving as complete a reduction of rhenium to the zero valent state as possible. This invention offers an efficient procedure for achieving this objective. It is believed that the principal benefit of the catalyst activation process of the present invention is the attainment of zero valent platinum which in turn catalyzes the reduction of rhenium to the zero valent state.
In the conventional process of activating reforming catalysts, such as platinum composited on alumina with rhenium, the catalyst is first oxidized with a flowing gas stream of an oxygen-containing gas such as air in combination with a halogen containing compound such as chlorine or hydrogen chloride at temperatures ranging from 850.degree. to about 1100.degree. F. and generally at temperatures ranging from 900.degree. to about 980.degree. F. for a period of time ranging from about 1 to about 25 hours and generally about 2 to about 10 hours. Thereafter, the oxidized metallic components of the catalyst are reduced by contact with a flowing stream of hydrogen at a single selected temperature ranging from about 700.degree. to about 950.degree. F. and generally about 850.degree. to about 900.degree. F. for a period of time ranging from about 0.5 to about 40 hours and generally for about 1 to about 10 hours.
During startup of a reforming operation, excessive amounts of light gases, e.g. methane and ethane, are normally produced unless proper pretreatment procedures are utilized. The light hydrocarbon gases, produced as a result of high hydrocracking activity of the catalyst, are particularly to be avoided during reforming since they decrease the yield of gasoline boiling products. Hydrocracking activity can be diminished if the catalyst is sulfided prior to contact with the feedstock. The presulfiding can be accomplished, for example, by passing a sulfur-containing gas, e.g. H.sub.2 S, through the catalyst bed. Other presulfiding treatments are known in the art.
In spite of the wide success of reforming catalyst, there continues to be a desire in the art, stimulated in part by the withdrawal from gasoline of alkyl lead compounds for ecological reasons, to further improve the octane quality of naphtha and gasolines without the aid of additives. While the above described catalyst systems and catalyst activation procedures are quite effective for reforming, further improvement in the performance of such catalyst is quite desirable.